An exemplary dye-sensitized solar cell was first announced by Gratzel research team, Switzerland (B. O'Regan, M. Gratzel, Nature 353, 737 (1991)). Since then, extensive research has been made with respect of a dye-sensitized solar cell.
Referring to FIG. 1, a dye-sensitized solar cell developed by M. Gratzel, et al. includes an metal oxide semiconductor electrode composed of a photosensitive dye capable of absorbing a visible ray to generate electron-hole pairs and nonocrystalline titanium oxide particles that can transfer the generated electrons. More specifically, electrons excited in the dye by irradiation of the visible ray are transferred through the titanium oxide particles, which is an n-type semiconductor, and the dye is regenerated by an electrochemical oxidation-reduction reaction of “I−/I3−” contained in a liquid electrolyte, thereby generating an electric current.
The above dye-sensitized solar cell may be manufactured in a cost-effective manner as compared to a monocrystalline silicon solar cell, an amorphous silicon solar cell, and a chemical compound semiconductor solar cell, all of which are well-known in the art. For that reason, attention has been paid to the dye-sensitized solar cell as a next-generation solar cell.
However, the dye-sensitized solar cell available at the present time is lower in photo-energy conversion efficiency than the silicon solar cell and the chemical compound semiconductor solar cell. This means that an increase in the photo-energy conversion efficiency is required to render the dye-sensitized solar cell practically usable.
As one of solutions to this problem, there is known a method in which light absorption efficiency of a metal oxide layer is sharply increased to obtain a high short-circuit current density (Jsc). To this end, it may be necessary to increase the adsorption amount of a dye to the metal oxide layer so that the dye may efficiently absorb incident light. In a typical dye-sensitized solar cell, the metal oxide layer is made porous to have an increased surface area. More specifically, such a porous titanium oxide layer is prepared by dispersing in ethanol 10 to 50 nm-sized anatase crystalline titanium oxide particles produced by hydrothermal synthesis of titanium alkoxide, adding an organic polymer or oligomer binder to produce a paste, and coating the paste on a transparent conductive substrate, followed by sintering. Such formation of the porous titanium oxide layer is performed by various kinds of general-purpose thin film formation methods, including a roller method, an air knife method, a blade method, a wire bar method, a spin method, and a spray method (see, for example, JP2006-286528A).
In this regard, where the titanium oxide particles have a size of 10 nm or less, they show an increase in specific surface area but may suffer from reduction in particle crystallinity and charge transportation efficiency. In contrast, where the size of the titanium oxide particles is large, the specific surface area thereof may be sharply reduced together with an accompanying decrease in the dye adsorption amount. This may result in sharp reduction in the photoelectric conversion efficiency of the dye-sensitized solar cell. Consequently, a porous titanium oxide layer composed of titanium oxide particles with a size of 10 to 20 nm is generally used to assure efficient light absorption.
Where the porosity of the porous titanium oxide layer is increased by reducing the density thereof, the increase of the specific surface area may be achieved, which brings out increase of the dye adsorption amount, but lowering of the charge transportation efficiency due to the increase in inter-particle resistance. In contrast, where the porosity of the porous titanium oxide layer is reduced by increasing the density thereof, the dye adsorption amount may be decreased.
In case of conventionally available titanium oxide nanoparticles, individual crystals forming polycrystalline structures of the nanoparticles have a reduced size and random crystal faces. Accordingly, the titanium oxide nanoparticles show a reduced electron transportation capability and an unsatisfactory conductivity.
As a consequence, a conventional solar cell with a titanium oxide layer composed of titanium oxide nanoparticles may have a limit in assuring maximized light absorption and greatly improving photoelectric conversion efficiency.